Thick film dielectric structures provide for superior resistance to dielectric breakdown, as well as a reduced operating voltage, compared to thin film electroluminescent (TFEL) displays e.g. as exemplified by U.S. Pat. No. 5,432,015. The thick film dielectric structure when it is deposited on a ceramic substrate will withstand higher processing temperatures than TFEL devices, which are typically fabricated on glass substrates. This increased high temperature tolerance facilitates annealing of phosphor films at higher temperatures to improve their luminosity. With these advantages and with recent advances in blue-emitting phosphor materials, displays have approached the luminosity and color coordinates required to achieve the technical performance of traditional cathode ray tube (CRT) displays. Nevertheless, further improvement in blue phosphor performance is required to simplify display design, to improve display reliability by lowering operating voltages and to keep pace with a trend towards higher color temperature specifications for displays.
Cerium-activated strontium sulphide phosphor materials have traditionally been used in electroluminescent displays for blue colors while manganese-activated zinc sulphide have been employed for red and green colors. The optical emission from these phosphor materials must be passed through an appropriate chromatic filter to achieve the necessary color coordinates for red, green and blue sub-pixels, resulting in a loss of luminance and energy efficiency. The manganese-activated zinc sulphide phosphor has a relatively high electrical to optical energy conversion efficiency of up to about 10 lumens per Watt of input power. Cerium-activated strontium sulphide phosphor has an energy conversion efficiency of 1 lumen per Waft, which is relatively high for blue emission. However, the spectral emission for these phosphors is quite wide, with spectral emission for the zinc sulphide-based phosphor material spanning the color spectrum from green to red and that for the strontium sulphide-based material spanning the range from blue to green. This necessitates the use of the optical filters. The spectral emission of the cerium-activated strontium sulphide phosphor can be shifted to some degree towards the blue by controlling the deposition conditions and activator concentration, but not to the extent required to eliminate the need for an optical filter.
Alternative blue phosphor materials having narrower emission spectra to provide the color coordinates required for a blue sub-pixel have also been developed. These phosphor materials include cerium-activated alkaline earth thiogallate compounds which provide good blue color coordinates, but exhibit relatively poor luminosity and stability. Higher luminosity and excellent color coordinates for blue pixels have been achieved with europium-activated barium thioaluminate phosphor materials.
More recently europium activated lanthanum thioaluminate has been shown to have utility as a blue light emitting electroluminescent phosphor. European Patent Application 1,148,111 discloses a thin film phosphor of this composition incorporated into a thick dielectric electroluminescent device for which a luminance of 300 candelas per m2 was realized when a 1 kHz electric field was applied across the device. While this application suggests the use of yttrium as a possible choice of cation for thioaluminate based phosphor materials, it does not suggest a partial substitution of yttrium for any metallic species in the phosphor.
Yttrium has been used as a co-activator to enhance the luminosity of rare earth-activated sulphide phosphor materials in thin film electroluminescent devices. U.S. Pat. Nos. 6,043,602 and 5,939,825 teach the use of yttrium as one of a series of co-activator species that may enhance the luminance of green and blue light-emitting calcium, strontium, barium or magnesium sulfide phosphor materials. These phosphors also include a metal dopant such as copper or lead, and in the case of U.S. Pat. No. 6,043,602, a halogen selected from fluorine, chlorine or iodine. U.S. Pat. No. 5,662,831 teaches a method for the preparation of an europium activated yttrium oxy-sulfide material as a cathodoluminescent phosphor material. The phosphor is provided as a slurry further pulverized to attain small particle sizes. These aforementioned patents simply disclose yttrium as a dopant or co-dopant in a binary alkaline earth sulfide or alternatively, as the sole cation of the host material. These aforementioned patents do not teach or suggest the use of yttrium for controlled partial substitution of the metallic species of the phosphor.
There remains a need in the art for new phosphors having improved properties that have use as thin films in electroluminescent displays. The present invention fulfills this and other needs.